text: Various types of aggregates can undergo different chemical reactions and swell inside concrete, leading to damaging expansive phenomena. 
  ;Alkali–silica reaction
  The most common are those containing reactive amorphous silica, that can react in the presence of water with the cement alkalis (KO and NaO). Among the more reactive siliceous mineral components of some aggregates are opal, chalcedony, flint and strained quartz. Silica (in fact silicic acid when hydrated) is easily dissolved by sodium hydroxide (NaOH) to form sodium silicate (), a strong desiccant with a high affinity for water. This reaction is at the core of the alkali–silica reaction (ASR): 
  Following this reaction, a hygroscopic and expansive viscous silicagel phase forms inside the affected aggregates which swell and crack from inside. In its turn, the volumetric expansion of the swollen aggregates damages the concrete matrix and extensive cracks propagate causing structural damages in the concrete structure. On the surface of concrete pavements, the ASR can also cause pop-outs, i.e. the expulsion of small cones (up to  in diameter), corresponding to aggregate particle size.
  A quite similar reaction (alkali-silicate reaction) can occur when clay minerals are present in some impure aggregates, and it may also lead to destructive expansion.
  ;Alkali–carbonate reaction
  With some aggregates containing dolomite, a dedolomitization reaction, also known as alkali-carbonate reaction (ACR), can occur where the magnesium carbonate () reacts with the hydroxyl ions () and yields magnesium hydroxide (brucite, ) and a carbonate ion (). The resulting expansion caused by the swelling of brucite can cause destruction of the material: 
  Often the alkali–silicate reaction and the dedolomitization reaction are masked by a much more severe alkali–silica reaction dominating the deleterious effects. Because the alkali-carbonate reaction (ACR) is often thwarted by a coexisting ASR reaction, it explains why ACR is no longer considered to be a major detrimental reaction. 
  ;Pyrite oxidation
  Far less common are degradation and pop-outs caused by the presence of pyrite (), a  disulfide (S-S) very sensitive to oxidation by atmospheric oxygen, that generates expansion by forming less dense insoluble iron oxides (), iron oxy-hydroxides (FeO(OH), or ·n) and mildly soluble gypsum (·2). 
  When complete (i.e., when all  ions are also oxidized into less soluble  ions), pyrite oxidation can be globally written as follows: 
  The sulfuric acid released by pyrite oxidation then reacts with portlandite ()) present in the hardened cement paste to give gypsum: 
  When concrete is carbonated by atmospheric carbon dioxide (), or if limestone aggregates are used in concrete,  reacts with calcite () and water to also form gypsum while releasing  back to the atmosphere: 
  The dihydrated gypsum is relatively soluble in water  at room temperature and thus mobile. It can easily be leached by infiltration water and can form efflorescences on the concrete surface while the insoluble ·n remain in place around the grains of oxidized pyrite they taint in red-ocre.